An alternator typically comprises a rotor mounted on a rotating shaft and disposed concentrically relative to a stationary stator. The rotor is typically disposed within the stator. However, the stator may be alternatively positioned concentrically within the rotor. An external energy source, such as a motor or turbine, commonly drives the rotating element, directly or through an intermediate system such as a pulley belt. Both the stator and the rotor have a series of poles. Either the rotor or the stator generates a magnetic field, which interacts with windings on the poles of the other structure. As the magnetic field intercepts the windings, an electric field is generated, which is provided to a suitable load. The induced electric field (which is commonly known as a voltage source) is typically applied to a rectifier, sometimes regulated, and provided as a DC output power source. The induced current is typically applied to a rectifier, sometimes regulated, and provided as a DC output power source. In some instances, a regulated DC output signal is applied to a DC to AC inverter to provide an AC output.
Conventionally, alternators employed in motor vehicle applications typically comprise: a housing, mounted on the exterior of an engine; a stator having 3-phase windings housed in the housing, a belt-driven claw-pole type (e.g. Lundell) rotor rotatably supported in the housing within the stator. However, to increase power output the size of the conventional alternator must be significantly increased. Accordingly, space constraints in vehicles tend to make such alternators difficult to use in high output, e.g. 5 KW, applications, such as for powering air conditioning, refrigeration, or communications apparatus.
In addition, the claw-pole type rotors, carrying windings, are relatively heavy (often comprising as much as three quarters of the total weight of the alternator) and create substantial inertia. Such inertia, in effect, presents a load on the engine each time the engine is accelerated. This tends to decrease the efficiency of the engine, causing additional fuel consumption. In addition, such inertia can be problematical in applications such as electrical or hybrid vehicles. Hybrid vehicles utilize a gasoline engine to propel the vehicle at speeds above a predetermined threshold, e.g. 30 Kph (typically corresponding to a range of RPM where the gasoline engine is most efficient). Similarly, in a so-called “mild hybrid,” a starter-generator is employed to provide an initial burst of propulsion when the driver depresses the accelerator pedal, facilitating shutting off the vehicle engine when the vehicle is stopped in traffic to save fuel and cut down on emissions. Such mild hybrid systems typically contemplate use of a high-voltage (e.g. 42 volts) electrical system. The alternator in such systems must be capable of recharging the battery to sufficient levels to drive the starter-generator to provide the initial burst of propulsion between successive stops, particularly in stop and go traffic. Thus, a relatively high power, low inertia alternator is needed.
In general, there is in need for additional electrical power for powering control and drive systems, air conditioning and appliances in vehicles. This is particularly true of vehicles for recreational, industrial transport applications such as refrigeration, construction applications, and military applications.
For example, there is a trend in the motor vehicle industry to employ intelligent electrical, rather than mechanical or hydraulic control and drive systems to decrease the power load on the vehicle engine and increased fuel economy. Such systems may be employed, for example, in connection with steering servos (which typically are active only a steering correction is required), shock absorbers (using feedback to adjust the stiffness of the shock absorbers to road and speed conditions), and air conditioning (operating the compressor at the minimum speed required to maintain constant temperature). The use of such electrical control and drive systems tends to increase the demand on the electrical power system of the vehicle.
Similarly, it is desirable that mobile refrigeration systems be electrically driven. For example, driving the refrigeration system at variable speeds (independently of the vehicle engine rpm) can increase efficiency. In addition, with electrically driven systems the hoses connecting the various components, e.g. the compressor (on the engine), condenser (disposed to be exposed to air), and evaporation unit (located in the cold compartment), can be replaced by an electrically driven hermetically sealed system analogous to a home refrigerator or air-conditioner. Accordingly, it is desirable that a vehicle electrical power system in such application be capable of providing the requisite power levels for an electrically driven unit.
There is also a particular need for a “remove and replace” high power alternator to retrofit existing vehicles. Typically only a limited amount of space is provided within the engine compartment of the vehicle to accommodate the alternator. Unless a replacement alternator fits within that available space, installation is, if possible, significantly complicated, typically requiring removal of major components such as radiators, bumpers, etc. and installation of extra brackets, belts and hardware. Accordingly, it is desirable that a replacement alternator fit within the original space provided, and interfaces with the original hardware.
In general, permanent magnet alternators are well known. Such alternators use permanent magnets to generate the requisite magnetic field. Permanent magnet generators tend to be much lighter and smaller than traditional wound field generators. Examples of permanent magnet alternators are described in U.S. Pat. No. 5,625,276 issued to Scott et al on Apr. 29, 1997; U.S. Pat. No. 5,705,917 issued to Scott et al on Jan. 6, 1998; U.S. Pat. No. 5,886,504 issued to Scott et al on Mar. 23, 1999; U.S. Pat. No. 5,92,611 issued to Scott et al on Jul. 27, 1999; U.S. Pat. No. 6,034,511 issued to Scott et al on Mar. 7, 2000; and U.S. Pat. No. 6,441,522 issued to Scott on Aug. 27, 2002.
Particularly light and compact permanent magnet alternators can be implemented by employing an “external” permanent magnet rotor and an “internal” stator. The rotor comprises a hollow cylindrical casing with high-energy permanent magnets disposed on the interior surface of the cylinder. The stator is disposed concentrically within the rotor casing, and suitably comprises a soft magnetic core, and conductive windings. The core is generally cylindrical width an axially crenellated outer peripheral surface with a predetermined number of equally spaced teeth and slots. The conductive windings (formed of a suitably insulated electrical conductor, such as varnished copper motor wire), are wound through a respective slot, outwardly along the side face of the core around a predetermined number of teeth, then back through another slot. The portion of the windings extending outside of the crenellation slots along the side faces of the core are referred to herein as end turns. Rotation of the rotor about the stator causes magnetic flux from the rotor magnets to interact with and induce current in the stator windings. An example of such an alternator is described in, for example, the aforementioned U.S. Pat. No. 5,705,917 issued to Scott et al on Jan. 6, 1998 and U.S. Pat. No. 5,92,611 issued to Scott et al on Jul. 27 1999.
The power supplied by a permanent magnet generator varies significantly according to the speed of the rotor. In many applications, changes in the rotor speed are common due to, for example, engine speed variations in an automobile, or changes in load characteristics. Accordingly, an electronic control system is typically employed. An example of a permanent magnet alternator and control system therefore is described in the aforementioned U.S. Pat. No. 5,625,276 issued to Scott et al on Apr. 29, 1997. Examples of other control systems are described in U.S. Pat. No. 6,018,200 issued to Anderson, et al. on Jan. 25, 2000. Other examples of control systems are described in commonly owned co-pending U.S. patent application Ser. No. 10/860,393 by Quazi et al, entitled “Controller For Permanent Magnet Alternator” and filed on Jun. 6, 2004. The aforementioned Quazi et al application is hereby incorporated by reference as if set forth verbatim herein.
The need to accommodate a wide range of rotor speeds is particularly acute in motor vehicle applications. For example, large diesel truck engines typically operate from 600 RPM at idle, to 2600 RPM at highway speeds, with occasional bursts to 3000 RPM, when the engine is used to retard the speed of the truck. Thus the alternator system is subject to a 5:1 variation in RPM. Light duty diesels operate over a somewhat wider range, e.g. from 600 to 4,000 RPM. Alternators used with gasoline vehicle engines typically must accommodate a still wider range of RPM, e.g. from 600 to 6500 RPM. In addition, the alternator must accommodate variations in load, i.e., no load to full load. Thus the output voltage of a permanent magnet alternator used with gasoline vehicle engines can be subject to a 12:1 variation. Accordingly, if a conventional permanent magnet alternator is required to provide operating voltage (e.g. 12 volts) while at idle with a given load, it will provide multiples of the operating voltage, e.g. ten (10) times that voltage, at full engine RPM with that load, e.g. 120 volts. Where the voltage at idle is 120 V, e.g. for electric drive air conditioning, or communications apparatus, the voltage at full engine RPM would be, e.g. 1200 volts. Such voltage levels are difficult and, indeed, dangerous to handle. In addition, such extreme variations in the voltage and current may require more expensive components; components rated for the high voltages and currents produced at high engine RPM (e.g. highway speeds) are considerably more expensive, than components rated for more moderate voltages.
Various attempts to accommodate the wide range of output voltages from permanent magnet alternators have been made. For example, the aforementioned Scott et al U.S. Pat. No. 5,625,276, describes a controller that selectively activates individual windings to achieve a desired output. The windings may be connected in a fully parallel configuration to provide high current at relatively low voltage levels, or in series to provide high voltage capacity. As drive RPM increases, individual windings are, in effect, disconnected from the operative circuit to control output voltage and/or current. However, particularly in compact high power, high speed ratio applications such as motor vehicles, the switching transitions between windings have deleterious effects, especially at the high end of the RPM range.
Other attempts have involved controlling the RPM of the alternator, and thus its voltage, independently of the engine RPM. An example of such an attempt is described in U.S. Pat. No. 4,695,776, issued Sep. 22, 1987 to Dishner. These solutions tend to involve mechanical components that are large, require maintenance and are subject to wear.
Other attempts have involved diverting a portion of the magnetic flux generated in the alternator to modulate output voltage. An example of a system is described in U.S. Pat. No. 4,885,493 issued to Gokhale on Dec. 5, 1989. Flux diversion, however, typically requires additional mechanical components and can be slow to react.
Rectification and regulation can be effected as a single process using a switching bridge (e.g. SCR bridge) with phase angle control of duty cycle. The bridge includes respective control switching devices (e.g. SCRs) that are selectively actuated to provide conduction paths between the input and output of the bridge. In essence, each half cycle (irrespective of polarity) of the AC signal produces a pulse of a predetermined polarity (typically positive) at the output of the bridge. The duration and timing of the conduction perhaps controls the output of the bridge. Such switching bridges may be “half controlled”, comprising a respective controlled switching device (e.g. SCR) and diode for each phase, or “full controlled”, comprising for each phase two switching devices (e.g. SCRs), one for each polarity.
Conventionally, the switching devices in the bridge are actuated in accordance with “phase angle control of duty cycle” to provide a predetermined voltage output level. Trigger signals to the switching devices are generated by a controller that detects zero crossings in the respective phases of the AC signal and generates the trigger signal accordingly (typically after a delay nominally corresponding to a predetermined phase angle in the AC signal, and, concomitantly with a desired DC output level). More particularly, in a conventional system, when a zero crossing is detected in a particular phase, the controller delays by a time period corresponding to the desired duty cycle (which, in turn, corresponds to the desired output voltage level). The delay is typically engendered by a one-shot or conventional timing circuit. For example, a capacitor is charged with current when the voltage across the capacitor exceeds a reference voltage, a trigger to the SCR associated with the phase is generated, and the capacitor discharged. In response to the trigger signal, the SCR turns on (is rendered conductive) and remains on until the current through it goes to zero, at which point it is rendered nonconductive until the next trigger signal. The cycle repeated in response to the next zero crossing of the appropriate polarity.
In a half controlled system, phase angle control of the output duty cycle is effected by selective actuation of the controlled switching devices during their associated half cycle of the AC signal; the diode segments of the legs are rendered conductive during the entire associated (opposite polarity) half cycle of the phase. The range of output signals that can be generated from a given AC signal level (and thus range of input AC signals) is thus limited, as compared to a full controlled system.
When full control is provided, the SCRs are each associated with a particular half cycle (polarity) of an associated phase. A trigger signal is generated in response to (e.g. after the phase delay) the zero crossing beginning the associated half cycle of the phase. Accordingly, provisions must be made to differentiate between positive going and negative going zero crossings.
When a switching bridge (e.g. SCR bridge) and phase angle control of duty cycle is used in conjunction with an AC power source that varies in magnitude and changes alternating frequency very rapidly (as in the case of an motor vehicle alternator) the variations in voltage output and ripple contents can be particularly significant. This is particularly true in full controlled systems. The variations in ripple contents in the output of the bridge can produce unacceptable output ripple harmonics and require extensive filtering. For example, the outputs of many alternators are not a uniform sine wave. Non-uniformities in amplitude and duration often occur between half cycles, and between phases of the AC input signal to the bridge, and are reflected in the outputs of the portions (legs) of the bridge circuitry associated with the respective phases. Such distortions and non-uniformities in the alternator output can occur for any of a number of reasons, such as, for example, variations in the placement of the winding turns relative to each other and, in the case of permanent magnet alternators, relative to the magnets. Further variations in the outputs of the portions (legs) of the bridge associated with the respective phases (due to, e.g. tolerances, temperature, etc) in component values between the circuitry associated with the various phases, cyclic change in frequency due to engine cylinder firing, variations in the magnetic air gap, variations of the saturation of the stator teeth as the magnet progresses etc.
In addition, the output of the generator often includes spurious components (e.g. spikes) that can be mistaken for zero crossings by the detector circuitry.
Thus, there is a need for a relatively inexpensive and efficient control system using relatively rugged semiconductors (such as SCRs) that can accommodate wide variations in the frequency and amplitude of an AC source (e.g. alternator), while minimizing output ripple harmonics and filtering requirements.
In some applications there may be relatively long lengths of electrical cable connecting the output of the converter to the load. For example, the cabling between the converter and battery (load) can be sufficient to cause a voltage drop between the converter and battery
There are also a number of other factors that can affect the operation of alternator systems in the. For example, the operation of alternator systems can be significantly affected, and sometimes disabled, by the temperature of the system components. There is a need for an alternator control including mechanisms for detecting temperatures harmful to the operation of the alternator system.
In alternator systems used to charge batteries, battery temperature has a direct impact on the optimal battery charging voltage and battery sulfation is a major contributor to shortened battery life. There is a need for alternator charging systems (particularly in motor vehicle applications) that can dynamically adjust output for optimized charging voltage and dynamically handle battery sulfation.
There is a need for an alternator charging system including a mechanism for intelligent control, (e.g. microprocessor), providing for example: monitoring electrical system performance; providing electrical system protection; and field adjustment of system operating parameters.
The stator of a conventional high current motor vehicle alternator is constructed with conductors of large cross sectional area effectively connected in series. Several problems can exist with this winding method. For example: because of the low number of turns (in some instances only a single turn) per pole phase coil, it is difficult or impossible to make a small change in design output voltage by changing the number of turns of the phase pole coil; the large cross sectional area of the conductors make the stator difficult to wind; and a short circuit between coils will typically burn out the entire stator and may stall the alternator, resulting in possible damage to the drive system or overloading the vehicle engine.
In general, permanent magnet alternators incorporating a predetermined number of independent groups of windings, wound through slots about predetermined numbers of teeth where the power provided by each group is relatively unaffected by the status of the other groups are known. For example, such an alternator is described, together with a controller therefor, in U.S. Pat. No. 5,900,722 issued to Scott et al. on May 4, 1999. In the alternator described in U.S. Pat. No. 5,900,722, the number of groups of windings was equal to an integer fraction of the number of poles, and the controller circuit selectively completed current paths to the individual groups of windings to achieve a desired output.
However, there remains a need for a compact high power alternator wherein a desired output voltage can be achieved by changing the number of terms of the phase pole coil, that is relatively easy to wind, and minimizes the consequence of short circuits, while at the same time facilitating cooling. There is also a need for a converter that can accommodate such an alternator.